Question

Using Delphi 7 I am reading from a serial port.

The read is always preceded by a write which triggers the h/w to measure from a sensor and write something for me to read (and there is always something to read).

I have two possibilities: manually enter a command and click a button to write that to the serial port (read model or f/w version, etc) or click a button to loop reading measurements until a stop button is pressed. These both use the same internal functions, so the code looks something like this:

WriteSerial('?model');
SerialData := ReadSerial();  // returns string
WriteSerial('?fw');
SerialData := ReadSerial();

and

while stopButtonNotPressed do
begin
  WriteSerial('?data');
  SerialData := ReadSerial();
  Memo1.Lines.Add(SerialData );
end;

The first variant (manually entering a command & pressing a button) is always successful, no matter how quickly or slowly I enter commands (hold down button for repeat), where are the second goes

pass
fail
pass
pass
fail
pass
pass
fail
... add infinitum

adding calls to sleep produces nothing, but trying to debug, I found that if I add a modal dialog box MsgDialog, 'Please close this dialog...', mtInfo, [mrOK]); to the loop, then it no longer fails.

Now, it doesn’t look like timing (else surely adding Sleep(2000); to the loop would make it pass & does not, so why does pressing a button on the main form or the modal dialog cause it to succeed?

Btw, the h/w user guide says nothing of CTS / RTS, and the sole code example provide also does not.

Note: if I manually enter ?data repeatedly it never fails …

Any ideas?

Answer 1

Your serial devices need time to react, so obviously you need a break for the device to catch up. When you use the keyboard to push the button you’re providing the brake it needs because the keyboard repeat isn’t all that fast.

As you say Sleep(2000) should provide plenty of “break”, but there are two other potential problems you’ll need to take care of:

• Serial communication isn’t necessary buffered: Sleep(2000) might be too long!
• The serial library you’re using might be using windows messages to process incoming bytes. Sleep() inhibits the message pump, so no more messages flow towords your application

Try “sleeping” using something like this:

procedure BusyWait(ms: Cardinal);
var StopAt: TDateTime;
begin
  StopAt := Now + EncodeTime(0, 0, ms div 1000, ms mod 1000);
  while StopAt > Now do
  begin
   Application.ProcessMessages;
   Sleep(50); // per Michał Niklas's suggestion, to keep the CPU from reaching 100%
  end;
end;

This routine will wait, but it’ll keep the message pump going, allowing your serial library to receive messages. If that’s the problem…